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Surprising Asymmetry Seen in Kaon Decays

CHICAGO--Once again, nature is teasing particle physicists. The Standard Model of subatomic particles, a body of theory that has survived several close shaves over the past few years, suggests that a lopsidedness in the laws of physics called direct CP violation should be small. Now data from millions of particle decays in the Tevatron particle accelerator at the Fermi National Accelerator Laboratory (Fermilab) near here point to a much larger asymmetry. Either the Standard Model is showing a crack--something theorists have long hoped for--or they have been applying the model incorrectly. Either way, "it's a shocking result," says Bruce Winstein, a University of Chicago physicist on the experiment, called KTeV.

For more than 30 years, CP violation has been known in a so-called indirect form: a certain long-lived particle, a type of kaon called K-long, decayed about once out of 500 times into two pions instead of three. Theorists later came up with an explanation that is compatible with the Standard Model by assuming that a K-long consists of a slightly lopsided mixture of an ordinary neutral kaon and its antiparticle. The same explanation predicted that CP violation would still be evident, but smaller, in decays of the neutral kaons themselves. So experimenters went looking for such "direct" CP violation.

It took decades of improvements in detector technology--and the design of a clever experiment--to pick out slight differences in the way kaons and antikaons decayed into several combinations of pions. Because the team did a blind analysis of the data, they themselves did not know the answer until a few days before the University of Chicago's Peter Shawhan gave a seminar at Fermilab on 24 February. "It was one of the most remarkable moments I've ever experienced in a physics seminar," says Chris Quigg, a Fermilab theorist who is not a member of KTeV. "When he ripped the Post-It off" a viewgraph, revealing the value, "there was a quarter-note rest in the whole audience during which nobody breathed; and then a big gasp, collectively."

The number was far larger than suggested by earlier, sketchier results from the Tevatron. Although the asymmetry is several times larger than the Standard Model had suggested, it definitively rules out an explanation based on a postulated "superweak" force that has no place in the Standard Model.